In a known embodiment, remote action terminals are connected in parallel with telephone sets so as to take advantage of the infrastructure of the telephone network, and in particular of pre-existing telephone lines for transmitting remote action signals and telephone signals over the same telephone lines, but over separate frequency bands in order to avoid interaction.
Servers are connected to ports of the remote action network. Other ports of the remote action network are connected to first ends of telephone lines via couplers which are complementary to other couplers at the subscriber end whereby the remote action terminals are themselves connected in parallel with telephone sets at the other ends of the telephone lines.
In most applications, it is important for the servers to be able to verify the presence of the terminals and their ability to communicate under remote control in order to avoid sending messages pointlessly to a terminal which is disconnected or incapable of communicating adequately.
Steps are therefore taken to perform continuous checking so that both the servers and the terminals are informed of failures that could disturb or prevent information interchange.
This is preferably done with a high degree of security by using devices which are simple and reliable.